The present invention relates to a high-speed field effect transistor.
A MISFET (i.e., Metal Insulator Semiconductor Field Effect Transistor) using a substrate of a compound semiconductor such as GaAs or InP is shown in section in FIG. 1(a). In FIG. 1(a): reference numeral 1 indicates a p-GaAs substrate; letter 2 an n-GaAs layer; letter 3 an insulating film; letter 4 an interface state layer; letter 5 a gate electrode; letter 6 a source electrode; and letter 7 a drain electrode. The MISFET thus composed is a device which is enabled to conduct operations of inversion type. When a positive voltage is applied to the gate electrode 5, more specifically, an electron inversion layer 8 is formed in the interface between the insulating film 3 and the p-GaAs substrate 1. Since the inversion layer is used as a channel, the MISFET has a possibility for a high-speed device. As a matter of fact, however, as shown in the band diagram of FIG. 1(b), there is formed in the interface between the insulating film 3 and the p-GaAs substrate 2 the layer 4 (which will be called the "interface state layer 4") having a thickness of 10 to 30 .ANG., in which a remarkably high-density interface state of 10.sup.12 to 10.sup.13 /cm.sup.2 /eV is present in the vicinity of the energy gap center of the semiconductor. As a result, the Fermi level .psi..sub.SS of the interface state has a strong tendency of being pinned at the Fermi level .psi..sub.P of the semiconductor so that the surface potential .psi..sub.S is not fluctuated even by applying a positive voltage to the gate electrode to make it difficult to form an inversion layer.
Thus, the existing MISFET finds it difficult to use the inversion layer as the channel so that it cannot allow a large current to flow therethrough. This is because the high-density interface state layer is present so that the Fermi level of that interface state has a tendency to be pinned at that of the semiconductor. The MIS transistor described above is disclosed, for example, in T. Ito and Y. Sakai "The GaAs Inversion type MIS Transistors", Solid-State Electronics vol. 17, pp. 751 to 759 (1974).